"Children are going to get measles, there is no question, because there is a great big group - probably of the order of approaching two million children - who are susceptible who have not been immunised," she said.

With authorities struggling to contain the outbreak which has claimed more than 800 cases around Swansea, she said that outbreaks were "already occurring" in the north of England.

Last night Dr David Elliman reissued a warning that those born around the turn of the century who have not had the full MMR vaccine are most vulnerable.

"They are now starting secondary school so they are vulnerable, they are mixing with large numbers of children, so collectively there is a large group who are unimmunised," he told the BBC. "We would be very worried that that's the group we're going to see outbreaks in."

Meanwhile the father of a young leukaemia sufferer urged parents to vaccinate their children against measles as scientists warned of an outbreak in London. John Davidson's six-year-old son, Henry, has leukaemia and is at high risk of catching measles because his MMR vaccination is no longer effective as a result of his chemotherapy.

Mr Davidson said: "I'm keeping him at home until we know if there's an outbreak here. We live in north London and I'm fairly sure older children round here haven't been vaccinated and we just can't take that risk. When we do hear a child has died, it will be a child like Henry. He can't fight the disease the same way another child can."